The present invention relates to the utilization of a corrosion proof austenitic iron chromium nickel nitrogen alloy as a structural material for components being subjected to high mechanical loads under corrosive conditions.
Very high pressure pipes and tubings are used for example in chemical engineering, for the conduction of acid gas or for implantates in bone surgery. These parts require steels or alloys which are not only highly corrosion proof but have very high strength because of the high mechanical load it is being subjected to. The 0,2% offset yield strength (0,2-limit) respectively the yield strength (yield point) are the decisive parameter for determining the strength of the material. The construction engineer when designing certain parts requiring corrosion proof material will prefer those with high yield points in order to attain higher load capabilities or because of easier conditions of working. In other cases saving of material or weight or both may lead to thinner or smaller parts, which still have to be strong accordingly.
Austenitic stainless steel or steel alloys usually have favorable corrosion properties and are easier to work than ferritic steels. Since the austenitic structure is primarily stabilized through nickel, such steels are usually alloyed with more than 7% nickel; see for example DIN 17 440, the December 1972 issue and Steel and Iron Material (translated), Flyer 400-73, 4th edition December 1973. Moreover these steels have at least 16% chromium in order to guarantee sufficient passivity. Molybdenum and silicon are added in order to improve the resistance against pitting. Copper is added in order to increase the corrosion resistance by exposure to nonoxidizing acids (see e.g. Hourdremont Handbook of Special Steel Engineering (translated) Springer, Berlin 1956, pages 969,1176, and 1261 et seg.). Increased nickel contents up to about 50% increases the stress corrosion resistance; see for example Berg- und Huttenmannische Monatshefte 108, page 1/8 and 4 et seg.
Austenitic chromium nickel steels are disadvantaged by their relative socalled 0.2-limits. Through the addition of up to 3% tungsten the strength values can be increased (see for example the particular statement made by Houdremont on pages 899 et seg). Of more importance, however, is the solid solution hardening through the utilization of nitrogen. Thus, the guaranteed minimum values of the 0.2-limits of corrosion proof austenitic steel being only about 200 N/mm.sup.2 will be increased by alloying with 0.2% nitrogen resulting in an increase of up to 300 N/mm.sup.2 (see for ex. DIN 17440, steel 1.4429 with app. 17.5% chromium, 13% nickel, 3% molybdenum and 0.2% nitrogen). This increase in strength is, generally speaking, approximately proportional with the amount of nitrogen in solution. That increase in strength is however not sufficient for all requirements. Higher contents of up to the limit of solution, in the solid state being about 0.55% nitrogen, are difficult to add owing to the formation of nitrogen bubbles during the solidification build up blowing hole in the casting ingots. Therefore such higher nitrogen contents can be included only if the chromium content is increased to about 24% and if the manganese content is increased to about 5%. Thus, the DEW technical report 13, 1973, page 94-100 describes a steel having 24.5%, 16.8% nickel, 5.5% manganese, 3.2.% molybdenum, 0.16% niobium, 0.46% nitrogen. The guaranteed lowest value of the 0.2 limit with 510 N/mm.sup.2 is stated for a solution annealing temperature to be about 1100 degrees C. The values actually meaured on hot rolled sheet stock were around 615, 670, 725 N/mm.sup.2 for solution annealing temperatures amounting respectively to 1100,1050, and 1000 degrees C.
Steel of the kind referred to in the preceeding paragraph has the disadvantage that it is quite brittle even at temperatures as high as 1000 degrees C. Therefore they precipitate intermetallic phases, and consequently such steel has a relatively low rupture elongation less than about 30%. Moreover such steel is difficult to hot working (see e.g. the citation in the DEW report above, line 11 and also the TEW technical report 2 of 1976, page 159 et seg. as well as METALS ENGINEERING QUARTERLY of Feb. 1971, page 61, 62 and 63.
Another aspect to be considered is that the relatively high chromium and manganese contents are intimately connected with the introduction of nitrogen; this aspect entails a relatively high amount of nickel in order to avoid formation of delta ferrite and of intermetallic phases. All these aspects increase the cost of such material. On the other hand in most cases steel having only about 18% chromium, 12% nickel, and 2% molybdenum are in demand.
Of further significance towards optimizing the yield strength in nitrogen alloyd austenitic steel is the inclusion if niobium as a particular alloying component. It was found for example that aside from the already mentioned nitrogen caused solution hardening effect an additional yield point increase results from niobium owing to the precipitation if niobium containing chromium nitrides of the kind Nb.sub.2 Cr.sub.2 N.sub.2 also called the Z-phase. Thus, the portion of the 0.2-limit attributable to precipitation hardening in such steel which recrystallized through annealing at 1050 degrees C. will amount to only 90 N/mm.sup.2 at the most; see for example Thyssen Research, vol. 1 1969, page 10/20 and 14 et seg.
In order to avoid precipitation of less effective niobium nitrides as well as in order to avoid larger losses in nitrogen in the austenitic structure, this kind of all steel has a significantly lower niobium content as compared with the 7-fold amount of nitrogen which is in effect the stoichiometric ratio in the compound NbN.
The third possibility of strengthening i.e. in addition to precipitation and solution hardening, is a grain size reduction or grain-refinement as per ASTM Special Technical Publication, No. 369 of 1965, p. 175-179. After cold rolling and recrystallization annealing of an austenitic steel with approximately 18% chromium and 10% nickel which was not alloyed with nitrogen, a grain size of the number 12.5 in accordance with ASTM (app. 4 micrometers) was obtained. However, the 0.2 limit of only about 300 N/mm.sup.2 was attained therewith because both, the nitrogen solution hardening and the nitride precipitation hardening was missing. As compared with a coarser structure of this alloy with a grain size of app. 5.5 (ASTM), being about 50 micrometers and corresponding to the usual solution annealed condition of steels, the yield strength increase amounted to maximum 150 N/mm.sup.2 (see e.g. above recited paper, FIGS. 6-9 on page 178).
Scandinavian journal of metallurgy - vol. 6, 1977, pages 156-169 and 162 et seg. suggests a nitrogen alloyed austenitic steel with app. 22% chromium, 10% nickel, 0.27% nitrogen. After cold rolling and a recrystallization annealing it had a smallest grain size of about 10 micrometers (ASTM No. 10) and a 0.2-limit of at the most 490 N/mm.sup.2. Strong grain refining did, therefore, not occur. Also a precipitation hardening through chromium nitride could not be ascertained, so that the observed strength enhancement relied exclusively on superimposing nitrogen solution hardening upon grain-refinement (grain size reduction) which however was quite limited owing to still relatively large grains as actually observed.
In view of the corrosion property of the various nitrogen alloyed steels as discussed one should mention that the chromium content diminished to some extent in the austenite result through the formation of Cr.sub.2 N. This means that the passivity of the steel in the environment of the precipitated particles may be lost. A measure of this type of corrosion is the susceptibility of the steel with regard to grain decay. It was found that steel having app. 18% Cr and 10% Ni will only be prone to corrosion in this regard through annealing above 800 degrees C. whenever the nitrogen content is in excess of 0.27% (see e.g. STEEL AND IRON No. 93, 1973, pages 9-18 and 15 et seg.). As was mentioned earlier, larger amounts of nitrogen can be alloyed into austenitic steel only when the chromium content is increased. Since in accordance with a paper, Berg- und Huttenmannische Monatshefte (1979), page 508/514-515 and 509 et seg. the tendency for grain decay i.e. for intercrystalline corrosion in a nitrogen alloyed austenitic steel decreases with the chromium content, one cannot expect corrosion problems being attributed to nitrogen to have any significant consequence when used in small proportions in such alloys.